This invention relates to electronics and, more particularly, to variable attenuator circuits. Specifically, the invention is directed to a wideband microwave Field-Effect-Transistor (FET)-based variable attenuator, preferably fabricated as a microwave monolithic integrated circuit (MMIC), having an improved dynamic range of attenuation over a broad range of frequencies and also having optimal input/output impedance matching characteristics.
An attenuator is a device passing an input signal while operating to attenuate the signal by a precise amount. A variable attenuator allows the level of attenuation to be adjusted.
Gain control of amplifier cascades generally requires a variable attenuator circuit. Voltage-controlled variable attenuators have been widely used for automatic gain control circuits. In broadband microwave amplifiers, these attenuators are indispensable for temperature compensation of gain variation.
One type of voltage controlled variable attenuator is a FET variable absorptive attenuator which utilizes FETs as voltage-controlled resistors to adjust the attenuation. The basic mechanism of the circuit is the change in the low field resistance of a zero-biased FET controlled by gate voltage. An expression for the channel resistance of a FET in the linear region appears in Equation 3 of Barta, G. S., et al., "A 2 to 8 GHz Leveling Loop Using a GaAs MMIC Active Splitter and Attenuator," 1986 I.E.E.E. Microwave Circuits Symposium, pg. 75-79.
There are two known basic configurations of FET attenuators, T-type and Pi-type, the circuit schematics of which appear in FIG. 1. See Tajima, Y., et al., "GaAs Monolithic Wideband (2-28 GHz) Variable Attenuators," 1982 I.E.E.E. MTT-S Digest, pp. 479-481. Typically, three FETs are connected in T or Pi configuration, as shown in FIGS. 1A and 1B, respectively.
The electrical characteristics of each FET are expressed as a parallel combination of resistance and capacitance, as shown in the equivalent circuit schematics which appear in FIGS. 1C and 1D, where resistance is a varying value as a function of gate voltage. The value of resistance varies from the open-gate resistance to infinite resistance when the gate voltage is changed from the built-up voltage (positive) of the gate barrier to the pinch-off voltage (negative). On the other hand, capacitance is considered to be fairly constant with the gate voltage. The parasitic capacitance values are typically tenths of a picofarad.
At relatively low frequencies, when the effect of capacitance can be neglected, resistance(s) R.sub.1, in a series arm, and resistance(s) R.sub.2, in a shunt arm(s), must have a certain combination in order to both obtain a given attenuation and to meet the impedance matching conditions. In either the T or Pi configuration, a specified level of attenuation and optimum input/output matching are simultaneously achieved by a proper combination of resistances, R.sub.1 and R.sub.2, controlled by voltages applied to the gate terminals of the FETs.
Insofar as dynamic range of attenuation is concerned, the minimum attenuation, or insertion loss, is determined primarily by the minimum achievable value for resistance R.sub.1. For the same resistance R.sub.1, Pi circuits have less insertion loss than T circuits.
In this regard, several factors must be considered when using FETs for the series and shunt elements. FET widths for the series FETs must be chosen wide enough for low insertion loss at minimum attenuation, but small enough to limit parallel drain-to-source capacitance, so that isolation at relatively high frequencies is sufficient. The isolation is most dependent on the parallel drain-to-source capacitance of the series FETs. For low insertion loss, the value of resistance R.sub.1 can be reduced by increasing the gate width, but parasitic capacitance C.sub.1 will increase. Larger capacitance limits the dynamic range of attenuation at relatively high frequencies. In terms of the dynamic range, T circuits become advantageous over Pi circuits.
Considered in more detail, by employing series FETs with large gate widths, small gate lengths, and narrow source-drain spacings, the "ON" state insertion loss can be significantly reduced. Unfortunately, known FET attenuators typically exhibit high ON state insertion losses at relatively high frequencies. The effect of the drain-to-source parasitic capacitance of the series FETs upon the isolation increases markedly at these higher frequencies. Parasitic capacitances of the series FETs degrade the high-frequency performance, resulting in a larger minimum insertion loss and a more restricted maximum attenuation attainable as frequency increases. This severely limits the attenuation range at higher frequencies.
For example, Schindler, M. J., and Morris, A. M., "DC-40 GHz and 20-40 GHz MMIC SPDT Switches," 1987 I.E.E.E. Microwave and Millimeter-Wave Monolithic Circuits Symposium, pp. 85-88, particularly FIG. 2 on p. 86, discloses a single-pole-double-throw FET-based switch. The isolation provided by this switch continually decreases as frequency increases, as shown in FIG. 5 on page 87 of this article. This evidences that the parasitic capacitance dominates the operation of the circuit at relatively high frequencies, in spite of the incorporation of an artificial transmission line. Similarly, the attenuator manufactured by M/A-Com Advanced Semiconductor Operations of Lowell, Massachusetts incorporates inductive elements as disclosed in FIG. 2 of "DC to 20 GHz MMIC GaAs FET Matched Attenuator," Microwave Journal, March, 1986, p. 195. However, the dynamic range of attenuation at 20 GHz is half that at 2 GHz, as shown in FIG. 3 of this article. This demonstrates that the parasitic capacitance degrades the performance of the attenuator at relatively high frequencies, notwithstanding the inductive elements fabricated in the circuit. Therefore, it is desirable to provide an attenuator in which the effects of parasitic capacitance are reduced so that the dynamic range of attenuation at relatively high frequencies can be extended.
Insofar as input/output impedance matching is concerned, for amplifier stability, it is desirable that the attenuator provides constant source and load match irrespective of the attenuation value. The aforementioned Barta, et al., article discloses that feedback can be used to control input and output return loss as attenuation is changed, invariant of any process effects or differences between FET geometries. FIG. 2 of this article shows a reference attenuator cell where an operational amplifier adjusts the shunt FET gate voltage in response to an arbitrary voltage variation on the series FET gate, maintaining a 50-ohm environment. Unfortunately, this solution of the impedance matching problem, particularly near minimum and maximum attenuation, requires an additional circuit having a complexity which exceeds that of the attenuator.
Also, the use of FETs as switching elements is well documented. See McLevige, W.V., and Sokolov, V., "Microwave Switching With Parallel-Resonated GaAs FETS,"I.E.E.E. Electron Device Letters, Vol. EDL-1, No. 8, August, 1980, pp. 156-158. By connecting the source and drain of a FET in series with a transmission line, the gate can be used to pinch off the channel and switch the device to the "OFF" state. When the gate is biased at zero volts (the "ON" state), a small resistance is present between the source and drain. When the gate is biased beyond pinch-off (the OFF state), the source and drain are capacitively coupled. Resistive elements are also present. Isolation can be improved by parallel resonating the source-drain capacitance with an inductor. However, this is effective only over a narrow frequency band. In order to minimize the effect of the OFF state capacitance in a broadband switch, a shunt FET is inserted. When the switch is closed, the shunt FET is pinched-off and acts primarily as a shunt capacitance. When the switch is open, the series FETs are pinched-off and act primarily as small capacitances. This capacitance is essentially grounded through the shunt FET. Isolation is primarily provided by the shunt FET, particularly at relatively high frequencies, where the series FETs provide very little isolation. Unfortunately, this FET-based switch does not provide sufficient isolation at maximum attenuation over a broad band of frequencies.